Interactions with conspecifics which include copulation result in changes in the social behavior of male prairie voles (M. Ochrogaster), a species distinguished by a sociosexual organization observed in few mammals. These behavioral changes, which include preferences for their mate and nest site protection, likely involve the neuropeptide arginine-vasopressin (AVP.) The proposed experiments will determine via behavioral observations and histological analyses what type of sensory stimulation received during social interactions is critical for changes in the social behavior of this species, as well as for alterations in their forebrain AVP activity. Further, a specific role for the AVP projections from the bed nucleus of the stria terminalis and medial amygdala to the lateral septum for post-copulatory behavioral displays will be investigated using neurotoxic lesions of AVP cell bodies and site-specific antagonism of AVP receptors.